1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically, to an image forming apparatus that forms an image on a moving body using toner.
2. Description of the Related Art
Image forming apparatuses such as a copying machine, a printer, a facsimile, a plotter, and an MFP including at least one of them have been widely known. In these image forming apparatuses, in general, an electrostatic latent image is formed on a surface of a drum (hereinafter referred to as a “photosensitive drum” for the sake of convenience) having photosensitive properties, and toner is attached to the electrostatic latent image, whereby so-called developing is performed, and a “toner image” is obtained.
In an image forming apparatus, image density control as below is performed so that a stable image density is always obtained.
(1) A test pattern including a plurality of toner patches for toner density detection created with different image forming conditions (exposure power, charging bias, developing bias, and the like) is formed on a photosensitive drum so as to have different toner densities.
(2) A reflecting optical sensor which is an optical sensing unit receives a reflected light from each toner patch of the test pattern; and a toner density of each toner patch is calculated using the output of the reflecting optical sensor and a predetermined calculation algorithm.
(3) From the relation between the toner density of each toner patch and a developing potential obtained from the image forming conditions, a developing gamma γ (an inclination between a developing potential on the horizontal axis and a toner density on the vertical axis) and a development start voltage Vk (an x-intercept when a developing potential is represented on the horizontal axis (x-axis) and a toner density is represented on the vertical axis) are obtained.
(4) Based on the obtained developing gamma γ, the image forming conditions such as an exposure power, a charging bias, a developing bias, and the like are adjusted so that the developing potential provides an appropriate toner density.
However, in an image forming process of a multi-color image forming apparatus, a plurality of toner images corresponding to each color such as black, magenta, cyan, and yellow, for example, are primarily transferred to an intermediate transfer belt in a superimposed manner, and is then secondarily transferred to a recording sheet in a lump; and the plurality of secondarily transferred toner images are fixed to the recording sheet, whereby a multi-color image is formed.
In this image forming process, since adjustment deviation of the optical scanning device (exposing device) and a plurality of photosensitive drums corresponding to each color and a variation of the photosensitive drum and respective driving mechanisms that drives the intermediate transfer belt appear as color deviation in a color image as they were, color deviation control is also indispensable.
As a specific method of color deviation control, in general, a test pattern for positional deviation detection of each color such as black, magenta, cyan, and yellow is formed on an intermediate transfer belt, the position of the test pattern of each color is read by a reflecting optical sensor, a positional deviation amount is calculated from the reading results and fed back to a writing time of image information, and color deviation on a recording sheet is corrected. A moving direction of a toner image on the intermediate transfer belt is referred to as a “sub-direction” and a direction orthogonal to the sub-direction is referred to as a “main direction.”
Various reflecting optical sensors have been proposed (for example, see Japanese Patent Application Laid-open No. 1-35466, Japanese Patent Application Laid-open No. 2004-21164, Japanese Patent Application Laid-open No. 2002-72612, Japanese Patent No. 4154272, Japanese Patent No. 4110027). For example, examples of a conventional reflecting optical sensor include a 1-LED and 2-PD reflecting optical sensor including one light-emitting element and two light-receiving elements and a 2-LED and 1-PD reflecting optical sensor including two light-emitting elements and one light-receiving element.
In the 1-LED and 2-PD reflecting optical sensor, a light beam emitted from one light-emitting element to a test pattern forms one beam spots on an intermediate transfer belt. On the other hand, in the 2-LED and 1-PD reflecting optical sensor, light beams emitted from two light-emitting elements to a test pattern form two beam spots on approximately the same location on an intermediate transfer belt with a time difference. In any of the reflecting optical sensors, the size (spot diameter) of the beam spot was approximately 2 mm to 3 mm.
The respective test patterns for toner density detection and positional deviation detection are formed on an intermediate transfer belt so as to overlap a formation position of a beam spot in relation to a main direction and are moved in a sub-direction with movement of the intermediate transfer belt.
In this case, the beam spot and the test pattern need to overlap even if there are a mounting error of the reflecting optical sensor, a formation position error in the main direction of the beam spot resulting from deviation of a beam emission direction due to a mounting error of the light-emitting element, a formation position error of the test pattern, and a positional error in the main direction of the test pattern resulting from a skew of the intermediate transfer belt. Thus, the length of each test pattern in the main direction is set to be larger than the spot diameter.
For example, a test pattern for toner density detection includes a plurality of toner patches arrayed in a line along the sub-direction, and each toner patch has a length of approximately 10 mm in the main direction and a length of approximately 15 mm in the sub-direction. Further, a test pattern for positional deviation detection includes a plurality of linear patterns parallel and inclined to the main direction, and each linear pattern has a length of approximately 8 mm in the main direction and a length of approximately 1 mm in the sub-direction.
However, in an image forming apparatus including the conventional reflecting optical sensor, it is difficult to shorten the time necessary for detecting a toner density and a positional deviation.